If [ Int ( YourTable::YourField ) = YourTable::YourFIeld and YourTable::YourFIeld > 0 ]
You could also define this auto-enter calculation that makes it impossible to enter a negative or noninteger value:
Filter [ Self ; 9876543210 ]
Also clear the "do not replace existing value.." check box and if the user enters a value that is not a positive integer, it will auto-correct to become one that is. (This may be dangerous for many data entry scenarios.)
I applied the "if" as it I already had part of an If statement linked to my dialog box in my script!
It worked perfectly